The present invention relates to a control system for a plant, which uses a self-tuning regulator, and also relates to an air-fuel ratio control system for controlling, to a target value, an air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine.
An example of a control system for a plant, which uses a self-tuning regulator is described in Japanese Patent Laid-open No. 11-73206. FIG. 15 is a block diagram showing a general configuration of a control system using a self-tuning regulator 104 as shown in this publication. The self-tuning regulator 104 includes a parameter adjusting mechanism 105 and an inverse transfer function controller 106. The parameter adjusting mechanism 105 identifies model parameters (which will be hereinafter referred to also as xe2x80x9cself-tuning parametersxe2x80x9d) of a controlled object model obtained by modeling a controlled object (an engine system). The inverse transfer function controller 106 calculates a self-tuning correction coefficient KSTR by an inverse transfer function of a transfer function of the controlled object model by using the model parameters identified by the parameter adjusting mechanism 105. An air-fuel ratio detected by an air-fuel ratio sensor 17 is converted into a detected equivalent ratio KACT by a converting section 103, and the detected equivalent ratio KACT is supplied to the self-tuning regulator 104.
A target value calculating section 102 calculates a target air-fuel ratio coefficient KCMD (target equivalent ratio) corresponding to a target air-fuel ratio, and inputs the target air-fuel ratio coefficient KCMD into a fuel amount calculating section 101 and the inverse transfer function controller 106. The parameter adjusting mechanism 105 identifies the model parameters according to the detected equivalent ratio KACT and the self-tuning correction coefficient KSTR. The inverse transfer function controller 106 calculates a present value of the self-tuning correction coefficient KSTR according to the target equivalent ratio KCMD, the detected equivalent ratio KACT, and past values of the self-tuning correction coefficient KSTR. The self-tuning correction coefficient KSTR and the target equivalent ratio KCMD are input to the fuel amount calculating section 101. The fuel amount calculating section 101 calculates a fuel amount TOUT, that is, an amount of fuel to be supplied to an internal combustion engine (which will be hereinafter referred to also as xe2x80x9cenginexe2x80x9d) 1, using the target air-fuel ratio coefficient KCMD, the self-tuning correction coefficient KSTR, and other correction coefficients.
More specifically, the engine system as a controlled object is modeled into a controlled object model (DARX model (delayed autoregressive model with exogenous input)) defined by Eq. (1) shown below:
KACT(k)=b0xc3x97KSTR(kxe2x88x922)+r1xc3x97KSTR(kxe2x88x923)+r2xc3x97KSTR(kxe2x88x924)+r3xc3x97KSTR(kxe2x88x925)+s0xc3x97KACT(kxe2x88x922)xe2x80x83xe2x80x83(1)
where b0, r1, r2, r3, and s0 are the model parameters identified by the parameter adjusting mechanism 105. When a model parameter vector xcex8(k) having the model parameters as elements is defined by Eq. (2), shown below, the model parameter vector xcex8(k) is calculated from Eq. (3) shown below:
xcex8(k)T=[b0, r1, r2, r3, s0]xe2x80x83xe2x80x83(2)
xcex8(k)=EPS xcex8(kxe2x88x921)+KP(k)ide(k)xe2x80x83xe2x80x83(3)
where KP(k) is a gain coefficient vector defined by Eq. (4) shown below, and ide(k) is an identification error defined by Eq. (5), shown below. Further, EPS is a forgetting coefficient vector defined by Eq. (6), shown below. In Eq. (6), xcex5 is a forgetting coefficient which is set to a value between xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d:                               K          ⁢                      xe2x80x83                    ⁢                      P            ⁡                          (              k              )                                      =                              P            ⁢                          xe2x80x83                        ⁢                          ζ              ⁡                              (                k                )                                                          1            +                                                            ζ                  T                                ⁡                                  (                  k                  )                                            ⁢              P              ⁢                              xe2x80x83                            ⁢                              ζ                ⁡                                  (                  k                  )                                                                                        (        4        )            xe2x80x83ide(k)=KACT(k)xe2x88x92xcex8(kxe2x88x921)T(k)xe2x80x83xe2x80x83(5)
EPS=[1, xcex5, xcex5, xcex5, xcex5]xe2x80x83xe2x80x83(6)
In Eq. (4), P is a square matrix wherein the diagonal elements are constants and all the other elements are xe2x80x9c0xe2x80x9d. In Eqs. (4) and (5), xcex6(k) is a vector defined by Eq. (7), shown below, and having a control output (KACT) and control inputs (KSTR) as elements.
xcex6(k)T=[KSTR(kxe2x88x922), KSTR(kxe2x88x923), KSTR(kxe2x88x924), KSTR(kxe2x88x925), KACT(kxe2x88x922)]xe2x80x83xe2x80x83(7)
Further, the inverse transfer function controller 106 determines the control input KSTR(k) so that Eq. (8), shown below, holds:
KCMD(k)=KACT(k+2) xe2x80x83xe2x80x83(8)
By applying Eq. (1) to Eq. (8), the right side of Eq. (8) becomes:
KACT(k+2)=b0xc3x97KSTR(k)+r1xc3x97KSTR(kxe2x88x921)+r2xc3x97KSTR(kxe2x88x922)+r3xc3x97KSTR(kxe2x88x923)+s0xc3x97KACT(k)xe2x80x83xe2x80x83(8a)
Accordingly, the following equation (9), shown below is obtained from Eqs. (8) and (8a). The control input KSTR(k) is calculated from Eq. (9):
KSTR(k)=(1/b0)[KCMD(k)xe2x88x92r1xc3x97KSTR(kxe2x88x921)xe2x88x92r2xc3x97KSTR(kxe2x88x922)xe2x88x92r3xc3x97KSTR(kxe2x88x923)xe2x88x92s0xc3x97KACT(k)]xe2x80x83xe2x80x83(9)
That is, the inverse transfer function controller 106 calculates the control input KSTR(k) so that a deviation e(k) between a future equivalent ratio KACT(k+2) which will be detected two control cycles later, and the present value KCMD(k) of the target equivalent ratio, becomes xe2x80x9c0xe2x80x9d. The deviation e(k) is defined by Eq. (10), shown below:
e(k)=KACT(k+2)xe2x88x92KCMD(k)xe2x80x83xe2x80x83(10)
The characteristic of the controlled object model defined by Eq. (1) does not completely coincide with the characteristic of the actual controlled object, but includes a modeling error (the difference between the characteristic of the controlled object model and the characteristic of the actual controlled object). Further, the parameter adjusting mechanism 105 adopts a fixed gain algorithm. Accordingly, when the target equivalent ratio KCMD changes stepwise as shown in FIG. 16, the detected equivalent ratio KACT is influenced by the identification behavior of the model parameters due to the modeling error and the fixed gain algorithm, which sometimes results in an overshoot of the detected equivalent ratio KACT with respect to the target equivalent ratio KCMD.
Such overshoot causes a reduction in the purification rate of a catalyst provided in an exhaust system of the engine. This results in a deterioration of exhaust characteristics. Furthermore, depending on engine operating conditions, there is a possibility of causing an engine output surge wherein the engine driving force fluctuates.
Accordingly, it is an object of the present invention to provide a control system for a plant, wherein a plant such as the above-described engine system is properly controlled, using a self-tuning regulator. As a result, an output from the plant accurately coincides with a control target value even when the control target value changes stepwise.
It is another object of the present invention to provide an air-fuel ratio control system for an internal combustion engine which can properly control the air-fuel ratio of an air-fuel mixture to be supplied to the engine. As a result, the actual air-fuel ratio detected in an exhaust system of the engine accurately coincides with a target value even when the target value changes stepwise, thereby preventing a deterioration in the exhaust characteristic and the engine output surge.
To attain the first object, the present invention provides a control system for a plant, including identifying means (54) and control means (55). The identifying means (54) identifies model parameters (b0, r1, r2, r3, s0) of a controlled object model obtained by modeling the plant. The control means (55) calculates a control input (KSTR) to the plant so that an output (KACT) from the plant coincides with a control target value (KCMD), using the model parameters (b0, r1, r2, r3, s0) identified by the identifying means (54). The control means (55) includes self-tuning control input calculating means and damping control input calculating means. The self-tuning control input calculating means calculates a self-tuning control input (KSTRADP), using the model parameters (b0, r1, r2, r3, s0) identified by the identifying means (54). The damping control input calculating means calculates a damping control input (KSTRDMP) according to a rate of change in the output (KACT) from the plant, or a rate of change in a deviation (e) between the output (KACT) from the plant and the control target value (KCMD). The control input (KSTR) to the plant is calculated as the sum of the self-tuning control input (KSTRADP) and the damping control input (KSTRDMP).
With this configuration, the self-tuning control input is calculated using the model parameters identified by the identifying means, and the damping control input is calculated according to the rate of change of the output from the plant, or the rate of change of the deviation between the output from the plant and the control target value. Then, the control input to the plant is calculated as the sum of the self-tuning control input and the damping control input. Accordingly, the overshoot of the output from the plant with respect to the control target value can be prevented, and the follow-up characteristic to the control target value can be improved. The xe2x80x9cfollow-up characteristicxe2x80x9d means a performance of a controller, with respect to the state in which the output from the plant follows up the control target value.
Preferably, the self-tuning control input calculating means calculates the self-tuning control input so that a response characteristic of the deviation between the output from the plant and the control target value becomes a specified characteristic.
With this configuration, the self-tuning control input is calculated so that the response characteristic of the deviation between the output from the plant and the control target value becomes a specified characteristic. As compared to the case where the response characteristic is not controlled to become a specified characteristic, the damping gain of the damping control input can be enlarged to thereby obtain a greater effect of reducing the overshoot.
The present invention provides another control system for a plant, including identifying means (54) and self-tuning control input calculating means (55). The identifying means (54) identifies model parameters (b0, r1, r2, r3, s0) of a controlled object model which is obtained by modeling the plant. The self-tuning control input calculating means (55) calculates a self-tuning control input (KSTRADP) to the plant, using the model parameters (b0, r1, r2, r3, s0) identified by the identifying means (54), so that an output (KACT) from the plant coincides with a control target value (KCMD). In addition, the self-tuning control input calculating means (55) calculates the self-tuning control input (KSTRADP) so that a response characteristic of a deviation (e) between the output (KACT) from the plant and the control target value (KCMD) becomes a specified characteristic.
With this configuration, the self-tuning control input to the plant is calculated by the self-tuning regulator using the model parameters identified by the identifying means so that the response characteristic of the deviation between the output from the plant and the control target value becomes a specified characteristic. When the rate of change of the control target value is large in the self-tuning regulator, there is a tendency for the identification behavior of the model parameters to have an effect on the control input, causing an overshoot of the output from the plant with respect to the control target value. By calculating the self-tuning control input to the plant so that the response characteristic of the deviation between the output from the plant and the control target value becomes a specified characteristic, the overshoot of the output from the plant can be reduced, when the rate of change in the control target value is large.
To attain the second object, the present invention provides an air-fuel ratio control system for an internal combustion engine, including identifying means (54), an air-fuel ratio sensor (17) provided in an exhaust system of the engine, and control means (55). The identifying means (54) identifies model parameters (b0, r1, r2, r3, s0) of a controlled object model which is obtained by modeling the engine. The control means (55) controls the air-fuel ratio of an air-fuel mixture to be supplied to the engine so that the air-fuel ratio (KACT) detected by the air-fuel ratio sensor coincides with a target value (KCMD). The control means (55) includes self-tuning control input calculating means and damping control input calculating means. The self-tuning control input calculating means calculates a self-tuning control input (KSTRADP), using the model parameters (b0, r1, r2, r3, s0) identified by the identifying means. The damping control input calculating means calculates a damping control input (KSTRDMP) according to the rate of change in the detected air-fuel ratio (KACT) or the rate of change in a deviation (e) between the detected air-fuel ratio (KACT) and the target value (KCMD). The air-fuel ratio of the air-fuel mixture to be supplied to the engine is controlled using the self-tuning control input (KSTRADP) and the damping control input (KSTRDMP).
With this configuration, the self-tuning control input is calculated using the model parameters identified by the identifying means, and the damping control input is calculated according to the rate of change of the air-fuel ratio detected by the air-fuel ratio sensor or the rate of change of the deviation between the detected air-fuel ratio and the target value. The air-fuel ratio of the air-fuel mixture to be supplied to the engine is controlled using the self-tuning control input and the damping control input calculated above. Accordingly, the overshoot of the detected air-fuel ratio with respect to the control target value can be suppressed, and the follow-up characteristic to the control target value can be improved.
The present invention provides another air-fuel ratio control system for an internal combustion engine, including an air-fuel ratio sensor (17) provided in an exhaust system of the engine and air-fuel ratio control means (42). The air-fuel ratio control means (42) controls the air-fuel ratio of an air-fuel mixture to be supplied to the engine so that the air-fuel ratio (KACT) detected by the air-fuel ratio sensor coincides with a target value (KCMD). The air-fuel ratio control means (42) includes response specifying control term calculating means for calculating a response specifying control term (KSTRADP) so that the response characteristic of a deviation (e) between the detected air-fuel ratio (KACT) and the target value (KCMD) becomes a specified characteristic. The air-fuel ratio of the air-fuel mixture to be supplied to the engine is controlled using the response specifying control term (KSTRADP).
With this configuration, the response specifying control term is calculated so that the response characteristic of the deviation between the detected air-fuel ratio and the target value, becomes a specified characteristic, and the air-fuel ratio of the air-fuel mixture to be supplied to the engine is controlled using the response specifying control term calculated above. Accordingly, the overshoot of the detected air-fuel ratio with respect to the target value can be suppressed, thereby improving the purification rate of the catalyst and suppressing fluctuations in output from the engine. As a result, the exhaust characteristic can be improved and the engine output surge can be suppressed.
Preferably, the air-fuel ratio control means (42) further includes identifying means (54) for identifying model parameters (b0, r1, r2, r3, s0) of a controlled object model which is obtained by modeling the engine. The response specifying control term calculating means calculates the response specifying control term (KSTRADP) using the model parameters (b0, r1, r2, r3, s0) identified by the identifying means (54).
With this configuration, the model parameters of the controlled object model which is obtained by modeling the engine are identified, and the response specifying control term is calculated using the model parameters identified above. Accordingly, the model parameters reflect an operating condition of the engine and aging in characteristics of the engine, and an optimum value of the response specifying control term can be obtained irrespective of operating conditions of the engine and the aging in characteristics of the engine. As a result, a good follow-up characteristic of the air-fuel ratio to the target value can be maintained.
Preferably, the air-fuel ratio control means (42) further includes damping control term calculating means for calculating a damping control term (KSTRDMP) according to the rate of change in the detected air-fuel ratio (KACT) or the rate of change in the deviation between the detected air-fuel ratio and the target value. The air-fuel ratio of the air-fuel mixture to be supplied to the engine is controlled using the response specifying control term (KSTRADP) and the damping control term (KSTRDMP).
With this configuration, the damping control term is calculated according to the rate of change in the detected air-fuel ratio, or the rate of change in the deviation between the detected air-fuel ratio and the target value. Further, the air-fuel ratio of the air-fuel mixture to be supplied to the engine is controlled using the response specifying control term and the damping control term calculated above. Accordingly, the overshoot of the detected air-fuel ratio with respect to the target value can be further reduced.
Preferably, the damping control term calculating means calculates the damping control term (KSTRDMP) using a damping coefficient (KDAMP, KDAMPxe2x80x2), and sets the damping coefficient (KDAMP, KDAMPxe2x80x2) according to an operating condition of the engine.
With this configuration, the damping control term is calculated by using the damping coefficient, and the damping coefficient is set according to an operating condition of the engine. Accordingly, by setting the damping coefficient to a small value in an engine operating condition where the overshoot is unlikely to occur, the follow-up characteristic to the target value can be improved. On the other hand, by setting the damping coefficient to a large value in an engine operating condition where the overshoot is prone to occur, the overshoot can be reliably suppressed. As a result, a good exhaust characteristic can be obtained over a wide range of engine operating conditions.
Preferably, the response specifying control term calculating means changes the response characteristic by using a response specifying parameter (POLE), and sets the response specifying parameter (POLE) according to an operating condition of the engine.
With this configuration, the response characteristic is changed according to the response specifying parameter, and the response specifying parameter is set according to an operating condition of the engine. Accordingly, by increasing the response speed in an engine operating condition where the overshoot is unlikely to occur, the follow-up characteristic to the target value can be improved. On the other hand, by decreasing the response speed in an engine operating condition where the overshoot is prone to occur, the overshoot can be reliably suppressed. As a result, a good exhaust characteristic can be obtained over a wide range of engine operating conditions.